Mechanical arm



FIG. 1 is a front, left perspective view of a mechanical arm showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left view thereof;

FIG. 5 is a right view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines shown on the drawings depict portions of the mechanical arm that form no part of the claimed design. 

CLAIM The ornamental design for a mechanical arm, as shown and described. 